1. Field of the Invention
The present invention relates to methods for manufacturing silicon carbide semiconductor devices, and particularly to a silicon carbide semiconductor device having a trench and a method for manufacturing the same.
2. Description of the Background Art
Japanese Patent Laying-Open No. 7-326755 discloses a silicon carbide semiconductor device having a trench. This patent publication states that a gate thermal oxidation film has a larger film thickness on a bottom surface of a trench than the film thickness thereof on a side surface of the trench, so that a threshold voltage becomes low and breakdown voltage between the gate and the drain becomes high. It is also stated that the bottom surface of the trench corresponds to a carbon plane, which allows for fast oxidation rate, of hexagonal single-crystal silicon carbide, whereas the side surface of the trench corresponds to a plane perpendicular to this carbon plane and allowing for slow oxidation rate. Hence, by performing a thermal oxidation process once, a thermal oxidation film can be formed such that the thickness of the thermal oxidation film on the side surface of the trench is greatly different from the thickness of the thermal oxidation film on the bottom surface of the trench.
According to a study conducted by the present inventors, however, it was difficult to selectively increase the film thickness of a gate oxide film to a sufficient degree on a bottom surface of a trench merely by using the difference in oxidation rate of silicon carbide dependent on the crystal orientation.